The optically active .beta.-hydroxyester derivatives are utilized in industry as intermediates for preparing pharmaceutically active compounds, agricultural chemicals, and other fine chemicals.
As a process for preparing the optically active .beta.-hydroxyester derivative (which is an optically active .beta.-hydroxyester protected by a silyl group at its hydroxyl group), Mukaiyama aldol reaction which comprises reacting an aldehyde and a ketene silyl acetal in the presence of an optically active Lewis acid catalyst is known. As the Lewis acid catalyst, various organometallic compounds such as those containing titanium, tin, boron or copper as their central metals are known. However, these organometallic metal compounds have certain drawbacks.
For instance, organometallic compounds other than organic copper compounds, such as organic titanium compounds, organic tin compounds, and organic boron compounds, are unstable in water. Therefore, the reaction should be performed in a non-aqueous phase. The reaction in a non-aqueous phase requires complicated procedures and therefore is disadvantageous in industry.
In the use of an organic copper compound, it is required to employ an aldehyde having a Lewis basic functional group for causing asymmetric induction. Such requirement to employ the specific aldehyde compound as the starting compound is disadvantageous from the viewpoint of industrial production. See J. Am. Chem. Soc., 118, 5814 (1996).
The organic titanium compound which gives a high enantiomer excess at a relatively small amount further has drawbacks in that the preparation of a ligand required for the asymmetric induction is not easy and requires carcinogenic .beta.-aminonaphthalene as a starting compound. See J. Am. Chem. Soc., 116, 8837 (1994).
The organic tin compound is disadvantageous because it should be employed in a large amount in the reaction. See Tetrahedron, 49, 1761 (1993). Furthermore, the organic tin compounds are nowadays said to act as endocrine disruptors to give adverse effects to human bodies.
The organic boron compound is also disadvantageous because it should be employed in a large amount in the reaction. See Tetrahedron Lett., 33, 1729 (1992), and Bull. Chem. Soc. Jpn., 66, 3483 (1993).